1. Field of the Invention
The present invention relates to an ion accelerator for efficiently injecting ions generated in plasma to an ion linac, and a high-intensity direct ion injection method using this ion accelerator and an ion accelerator which is improved and injects still more efficiently ions, generated in a plasma-generating target by radiating a plasma-generating laser, into an ion linac by using the diffusion velocity of this plasma.
2. Description of the Related Art
Ion accelerators which inject ions generated in plasma to an ion linac such as an RFQ linac or a drift tube linac and accelerate the ions have been developed.
It is possible to use such an ion accelerator as a first-stage ion accelerator in an accelerator for cancer treatment, in an ion implantation accelerator for semiconductor production, and in a large-scale accelerator complex for physical experiments.
This ion accelerator will be described with reference to FIG. 11.
FIG. 11 is a plan for schematically showing the construction of a conventional ion accelerator 200.
As shown in FIG. 11, the conventional ion accelerator 200 mainly consists of an ion source 210, a beam line 220, and an ion linac 230.
Hereinafter, each major component of the conventional ion accelerator 200 will be described below.
As the ion linac 230, a well-known ion linac such as an RFQ linac described later, or a drift tube linac is used.
In FIG. 11, reference numeral 60 denotes a laser generator for generating a plasma-generating laser L, and reference numerals 62A and 62B denote mirrors guiding the plasma-generating laser L to an ion source 210.
In addition, reference numeral 70 denotes an analysis electromagnet for providing ions accelerated by the ion accelerator 200, for other applications such as the accelerator for cancer treatment, ion implantation accelerator for semiconductor production, or large-scale accelerator complex for physical experiments, which are described above.
Generally, an ion source is an apparatus wherein plasma with ions and electrons coexisting with each other is generated in a vacuum chamber by high-frequency power, laser heating, etc., and a high voltage is applied to the vacuum chamber to take out only ions from its inside, producing an ion beam.
The ion source 210 that is used for the conventional ion accelerator 200 shown in FIG. 11 comprises a plasma-generating target 212 which is subject to radiation of a plasma-generating laser L to generate the plasma, a focusing lens 214 which condenses the plasma-generating laser L at the plasma-generating target 212, a vacuum chamber 216 which contains the generated plasma, and an ion extraction electrode 218.
As shown in FIG. 11, the plasma-generating laser L generated by the laser generator 60 is radiated at the plasma-generating target 212 through the two mirrors 62A and 62B, and focusing lens 214 in the vacuum chamber 216 of the ion source 210 to generate the plasma by laser ablation.
Since the plasma generated in the vacuum chamber 216 is in the status in which ions and electrons coexist as described above, an ion beam is led to an adjoining beam line 220 by applying a negative voltage of several kV to several tens kV to the ion extraction electrode 218.
The beam line 220 comprises one or more ion beam focusing lenses 222 (two in FIG. 11), such as a solenoid type magnet or an Einzel electrostatic lens.
In addition, in order to control the status of an ion beam, a beam shape diagnostic tool 224 is often provided between the focusing lenses 222.
In the above construction, the basic operation of the conventional ion accelerator 200 will be described by using FIG. 11.
In the conventional ion accelerator 200, the plasma generation laser L is radiated at the plasma-generating target 212 to generate the plasma, ions extracted by the extraction electrode 218 from this generated plasma are injected into the ion linac 230 through the beam line 220.
At this time, it is possible to obtain the maximum values of the magnitude and gradient of an ion beam that suit the beam line 220 after the ion source 210 by adjusting the geometry and the applied potential gradient of the extraction electrode 218 which is an electrode for applying a high voltage.
In addition, the ion beam radius is expanded to large radius after the extraction by using a solenoid type magnet or the focusing lens 222 such as an Einzel electrostatic lens, travels with relatively low influence of Coulomb repulsion, is converged by means of the focusing lens 222 to the beam size of suitable injecting conditions for the ion linac 230, and is injected.
Next, as an example of the ion linac 230, as disclosed in Japanese Patent Laid-Open No. 7-111198, the well-known RFQ linac 230 will be supplementarily described by using FIGS. 12 and 13.
FIG. 12 is a cross sectional front view showing the construction of the RFQ linac 230.
FIG. 13 is a longitudinal sectional side view showing the construction of the RFQ linac 230.
The RFQ (Radio Frequency Quadrupole) linac 230 is mainly constituted by installing four vane electrodes 234 (or four rod electrodes), made to be perpendicular to each other, inside a conductive cylindrical container 232 whose inside is in vacuum.
A resonator comprises a cylindrical container 232 and vane electrodes 234, as shown in FIG. 13, high-frequency power is supplied through the high-frequency waveguide 238, and the vane electrodes 234 with end portions 234a in a wave form converge the ions and accelerates the ions in a direction of the central axis with a desired energy.
However, in the conventional ion accelerator with the above-described combination of the ion source, the beam line for transporting a low-energy ion beam, and the ion linac, the divergence of the beam by the Coulomb repulsion in the ion beam is large especially when a large-current ion source is used, and thus, only a part of the extracted ion beam can meet injection conditions of the ion linac, resulting a problem that only a small amount of ions to be accelerated.
In addition, since the amount of an ion beam current and the number of charges of generated ions, etc. largely change within a beam generating pulse of a duration of several xcexcs when a pulsed ion source with laser heating etc. is used as an ion source, it is very difficult to appropriately design a beam line while considering the Coulomb repulsion.
Furthermore, the conventional ion accelerator has a problem that it requires a complicated beam line including apparatuses such as a focusing lens.
An object of the present invention is to provide an ion accelerator where an amount of accelerable ions significantly increases by solving the above-described conventional problems, dramatically simplifying the combination of an ion source, a beam line, and an ion linac, and furthermore, further reducing the influence of Coulomb repulsion, and a direct ion injection method for efficiently injecting ions by using this ion accelerator and an ion accelerator which is improved and injects still more efficiently ions, generated in a plasma-generating target by radiating a plasma-generating laser, into an ion linac by using the diffusion velocity of this plasma.
In order to solve the problems, a first aspect the present invention is an ion accelerator comprising: a plasma-generating source; a vacuum chamber for extracting ions from plasma generated from the plasma-generating source; an ion linac, the plasma-generating source, vacuum chamber, and ion linac being connected in series, the vacuum chamber being installed near an ion entrance of the ion linac; and a high voltage power supply for boosting the vacuum chamber to a desired voltage, wherein ions are directly injected from the vacuum chamber to the ion linac.
Owing to such construction, since Coulomb repulsion is not generated because electrons with negative charges and ions with positive charges coexist in plasma, its influence is avoidable to the point just before an ion linac, and in consequence, the construction is simplified and an amount of accelerable ions also significantly increases.
According to a second aspect of the present invention is an ion accelerator wherein an injection slit is installed in an ion entrance of the ion linac.
Owing to such construction, it is possible when the divergence angle of the generated plasma is large to prevent bombardment with excess plasma onto an acceleration electrode of the linac, and discharge occurrence.
In addition, usually, since a strong high-frequency electric field is generated near an entrance of an ion linac, and hence few electrons passing the slit in this region can be injected to an acceleration channel of the linac, the ions and electrons are efficiently separated.
A third aspect of the present invention is an ion accelerator wherein the injection slit is adjustably installed in a radial direction of the ion entrance of the ion linac.
Owing to such construction, it becomes possible to perform positioning for the accurate centering of the injection slit with respect to the linac.
A fourth aspect of the present invention is an ion accelerator wherein the plasma-generating source is a plasma-generating target for generating plasma by a plasma-generating laser being radiated thereon.
Owing to such construction, since high-density plasma can be generated, it becomes possible to increase the intensity of accelerable ions.
A fifth aspect of the present invention is an ion accelerator wherein a focusing for the plasma-generating laser radiated to the plasma-generating target is installed in the vacuum chamber.
Owing to such construction, since the density of the plasma-generating laser on the plasma-generating target increases, it becomes possible to increase plasma generation efficiency.
A sixth aspect of the present invention is an ion accelerator wherein the focusing is installed so that it can move in three axes.
Owing to such construction, it becomes possible to adjust a focusing position of the plasma-generating laser on the plasma-generating target.
A seventh aspect of the present invention is an ion accelerator comprising a target positioning device having one or more mirrors and one or more centering lasers, which performs the alignment of the plasma-generating target with a focus of the plasma-generating laser.
Owing to such construction, it becomes possible to perform the accurate alignment of the plasma-generating target with the focus of the plasma-generating laser.
An eighth aspect of the present invention is an ion accelerator comprising a set of split type focusing lenses that are installed inside the ion linac and can concentrate the laser beam onto the target.
Owing to such construction, since the distance between the plasma-generating target and ion linac is reduced, it becomes possible to increase an amount of accelerable ions.
A ninth aspect of the present invention is an ion accelerator wherein the plasma-generating target is cylindrical so as to be rotatable.
Owing to such construction, although the plasma-generating target is damaged when the plasma-generating laser is radiated, it becomes possible to always obtain a good target surface without exchanging the plasma-generating target by rotating the plasma-generating target by a predetermined angle to change a radiated position.
A tenth aspect of the present invention is an ion accelerator wherein the vacuum chamber is boosted by the high-voltage power supply so that the injection energy of ions is a design injection energy of the ion linac.
Owing to such construction, since the voltage equivalent to the ion beam energy for satisfying the design condition of the ion linac is applied to the vacuum chamber with the generated plasma, only the ions having passed the slit are injected into the ion linac and accelerated.
At that time, since the ion beam is in a state just after being emitted from the plasma, the influence of Coulomb repulsion is very small, and hence, it is also possible to avoid the influence of the abrupt change of the number of ionic charges and the amount of current which changes within a pulse.
An eleventh aspect of the present invention is a direct ion injection method for using the ion accelerator according to any of claims 1 to 10, so as to directly inject ions generated from the plasma-generating source, from an ion entrance of an ion linac.
Then, since Coulomb repulsion is not generated because electrons with negative charges and ions with positive charges coexist in plasma, its influence is avoidable to the point just before an ion linac, and in consequence, the construction is simplified and an amount of accelerable ions also significantly increases.
A twelfth aspect of the present invention is an ion accelerator constituted by serially connecting a plasma-generating target for generating plasma by radiating a plasma generating laser, a vacuum chamber that extracts ions from plasma generated in the above-described plasma-generating target and is directly installed in an ion entrance of an ionic linac, and an ion linac, so that ions may be directly injected into the above-described ion linac by using the diffusion velocity of the plasma.
Owing to such constitution, it becomes possible to let the plasma-generating target get close to an acceleration electrode of the ion linac to a limit since it becomes unnecessary to install a vacuum chamber, to which a high voltage is applied and in which plasma is generated, through an insulated section with differing from the ion accelerators mentioned in the above-described first to eleventh aspects, and hence, it becomes possible to inject almost all of generated ions into the ion linac at the diffusion velocity of the plasma itself without applying a high voltage.
Therefore, it is possible to efficiently accelerate even a pulsed ion beam with large current, which is excited by a laser, by a simplified apparatus.
In this result, since Coulomb repulsion is not generated because electrons with negative charges and ions with positive charges are intermingled in plasma, its influence is avoidable just before the ion linac.
At that time, since the ion beam is immediately after being emitted from the plasma, the influence of the Coulomb repulsion is very small, and hence, it is also possible to avoid the influence of the abrupt change of the number of ionic charges and the amount of current that changes within a pulse.
In addition, usually, since a strong high-frequency electric field is generated inside an ion linac, and hence almost no injected electrons can stay in an acceleration channel of the linac, the ions and electrons are efficiently separated.
A thirteenth aspect of the present invention is an ion accelerator that is constituted so that a non-modulation section for extending the pulse width of ions may be formed in the acceleration electrode of the ion linac used for the above-described ion accelerator.
Owing to such construction, since an ion beam, which is separated and captured by the ion linac, is at comparatively low speed and has the velocity distribution of each ion particle that is the same extent of speed as that of the ion beam, it is possible to generate several tens xcexcs of pulse width with a several-meter long ion accelerator since ions spread in the axial direction of the ion accelerator by performing such design that an area where an accelerating electric field in the ion accelerator is not generated may become long.
According to a fourteenth aspect of the present invention, an ion accelerator has the construction that an injection slit is installed in the ion entrance of the above-described ion linac.
Owing to such construction, it is possible to prevent a discharge from being caused by excessive plasma striking an acceleration electrode of the linac when a divergence angle of the generated plasma is large.
In addition, usually, since a strong high-frequency electric field is generated near an entrance of an ion linac, and hence almost no electrons passing the slit in this region can be injected to an acceleration channel of the linac, the ions and electrons are efficiently separated.
A fifteenth aspect of the present invention is an ion accelerator having the construction that the above-described injection slit is adjustably installed in the radial direction of the ion entrance of the above-described ion linac.
Owing to such construction, it becomes possible to perform positioning for the accurate centering of the injection slit to the linac.
A sixteenth aspect of the present invention is an ion accelerator comprising a split type focusing lens that is installed inside the above-described ion linac and condenses a plasma-generating laser.
Owing to such construction, since the distance between the plasma-generating target and ion linac becomes short, it becomes possible to increase an amount of accelerable ions.
According to a seventeenth aspect of the present invention, an ion accelerator has the construction that the above-described beam-condensing unit is installed so that it can move in three axes.
Owing to such construction, it becomes possible to adjust a focusing position of the plasma-generating laser on the plasma-generating target.
An eighteenth aspect of the present invention is an ion accelerator comprising one or more mirrors, one or more centering lasers, and a target positioning device that performs the alignment of the plasma-generating target and a focus of the plasma-generating laser.
Owing to such construction, it becomes possible to perform the accurate alignment of the plasma-generating target and the focus of the plasma-generating laser.
A nineteenth aspect of the present invention is an ion accelerator where the above-described plasma-generating target is cylindrical so as to be rotatable.
Owing to such construction, although the plasma-generating target is damaged when a plasma-generating laser is radiated, it becomes possible to always obtain a good target surface without exchanging the plasma-generating target by changing a radiated position by rotating the plasma-generating target by a predetermined angle.
A twentieth aspect of the present invention is an ion accelerator constituted by using an RFQ linac or a drift tube type linac as the above-described ion linac.
Owing to such construction, it becomes possible to obtain an ion accelerator equipped with an ion linac suitable for ion acceleration.